The goal for the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) program is to provide improved spectral efficiency, reduced latency, and to enhance utilization of the radio resource to bring faster user experiences with reduced cost.
Since LTE is a Packet Switched (PS)-only radio technology that is initially likely to be supported only in packets, it is necessary to support backwards mobility with legacy Global System for Mobile communication (GSM), which is ubiquitous and may be assumed to exist everywhere. It was assumed that for inter-working with legacy Circuit Switched (CS) networks (such as GSM) operators would deploy Internet Protocol (IP) Multimedia Subsystem (IMS) networks. Specifically, Voice Call Continuity (VCC) was expected to be the technique used for handing over voice calls from LTE PS, using Voice Over Internet Protocol (VoIP) techniques, to legacy CS. However, it would also be desirable to de-couple IMS deployments from LTE deployments. In other words, operators would like to use their deployed CS infrastructure for voice calls while deploying LTE initially for high-speed PS services only. For this reason operators would like to see the initial release of LTE equipped with a feature (termed as “CS Fallback”) which would allow a multi-mode wireless transmit/receive unit (WTRU) (LTE+GSM and/or WCDMA) to use the LTE network for high-speed PS data traffic while reverting to legacy CS network for voice traffic without necessarily using any IMS features such as VCC.
In general, when the WTRU attaches to the Evolved Packet System (EPS) over the Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN) network the Non Access Stratum (NAS) layer Attach message will include a “CS Fallback Indicator” that would indicate to the network the need to attach the WTRU in the CS domain as well. The Mobile Management Entity (MME) would then perform the attachment on behalf of the WTRU to the CS domain before indicating the completion of the process in the Attach Accept message as shown below.
FIG. 1 is a diagram of the procedure 100 for the CS fallback in the EPS. Referring to FIG. 1, the wireless transmit receive unit (WTRU) 110 initiates the attach procedure by the transmission of an Attach Request message 120 (e.g., CS fallback indicator message to the MME 130). The CS fallback indicator 120 indicates that the WTRU 110 is capable and configured to use CS fallback. An EPS Attach procedure 140, such as the procedure described in 3GPP TS 23.401, is then performed.
The visitor location register (VLR) (not shown) is updated according to the combined GPRS/IMSI Attach procedure, if the Attach Request message 120 includes a Combined Update indicator. The VLR number is derived from the International Atomic Time (IAT). The MME starts the location update procedure 150 towards the new Mobile Switching Caller (MSC)/VLR 160 upon receipt of the first Insert Subscriber Data message (not shown) from the Home Subscriber Service (HSS) 170. This operation marks the WTRU 110 as EPS-attached in the MSC/VLR.
The MME 130 then sends a Location Update Request (new Location Area Identity (LAI), IMSI, MME address, Location Update Type) message 180 to the VLR 160. New LAI is determined in the MME 130 based on mapping information from the TA (not shown). A mapped LAI could be for either GERAN or UTRAN based on an operator configuration. The VLR 160 creates an association 190 with the MME 130 by storing MME address. The VLR 160 performs Location Updating procedure 192 in CS domain. The VLR 160 responds with Location Update Accept 194 (VLR temporary mobile subscriber identity (TMSI)) to the MME 130. The MME 130 sends an Attach Accept (e.g., location area (LA), VLR TMSI) message 196 to the WTRU 110. The existence of LA and VLR TMSI indicates successful attach to CS domain.
Existing high-level signaling for preparing and executing fallback procedures for Mobile Originating (MO) and Mobile Terminating (MT) voice calls in Active Mode has several unresolved issues. For example, one issue is that some networks may support voice in LTE (using IMS/VCC) and some might not. Depending on the capabilities of the network, the telephony client in the WTRU application processor must determine whether to route MO calls to the PS domain (and request a PDP context activation from the ESM sub-layer in the NAS) or to the CS domain (and request a call to be setup).
Another unresolved issue is how call control in the WTRU is handled. If the call originated in the CS domain within the WTRU (because the network did not support IMS), the call would typically be routed to the CC/MM protocols within the WTRU and the EMM sub-layer would be unaware of the request.
Alternatively, if the WTRU started by using PS for voice it would use a VoIP client application which would request an IP address from the ESM layer (EPS Session Management) in the NAS. The ESM layer in turn would contact the EMM (EPS Mobility Management) layer which would interact with the RRC to setup the connection. At this point the LTE network would order the WTRU to “fall back” to the CS domain. However once the WTRU “falls back” to the CS domain, the call is now routed to the CS domain within the WTRU and the MM/CC protocols are responsible for this connection.
It would be desirable to provide this indication of “fallback complete” to the CS voice client and the Call control/Mobility Management (CC/MM) protocols within the WTRU and transfer the call control from the VoIP client/ESM/EMM to the CS call client/CC/MM. Accordingly, there exists a need for an improved method and apparatus to enable fallback to the CS domain from the PS domain.